Antireflection structure formation method and antireflection structure

ABSTRACT

The present invention provides such a formation method that an antireflection structure having excellent antireflection functions can be formed in a large area and at small cost. Further, the present invention also provides an antireflection structure formed by that method. In the formation method, a base layer and particles placed thereon are subjected to an etching process. The particles on the base layer serve as an etching mask in the process, and hence they are more durable against etching than the base layer. The etching rate ratio of the base layer to the particles is more than 1 but not more than 5. The etching process is stopped before the particles disappear. It is also possible to produce an antireflection structure by nanoimprinting method employing a stamper. The stamper is formed by use of a master plate produced according to the above formation method.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 59/2008, filed on Jan. 4,2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antireflection structure formationmethod and an antireflection structure.

2. Background Art

According as ultrafine micro-fabrication technologies used in LSIproduction processes and micro-machining processes have been developedrecently, it has become possible to produce sub-wavelength gratings,which have periods shorter than the wavelength of light and which can beprocessed in nanometer scale. For example, as one of the sub-wavelengthgratings, there is proposed a non-reflective periodic structure (see,Hiroshi Toyota, Kogaku (Japanese Journal of Optics [in Japanese]),Optical society of Japan, vol. 32 (2003), pp. 489). The non-reflectiveperiodic structure has a surface provided with numerous smallprojections by which Fresnel reflection on the light-incident surface isreduced to ensure aimed antireflection properties. The Fresnelreflection is generally determined by inherent refractive index ofsubstance. However, in the non-reflective periodic structure, therefractive index is artificially set up with the surface structureformed more finely than the wavelength of incident light and is alsomade to vary continuously from the light-incident side to the substrateside, and thereby the aimed antireflection properties can be realized.This antireflection structure can reduce reflectance in a widewavelength range and in a wide incident angle range, and accordingly iseffectively used for optical elements or devices, such as lenses anddisplays, in which light reflection often becomes a large problem.

In order to obtain the above antireflection effect, the intervals amongthe projections must be several hundred nanometers or less and hence thefabrication in nanometer scale is required. In view of this, it isproposed to fabricate an antireflection structure by electron beamlithography or by laser beam interference exposure method (see, P.Lalanne, et. al., Nanotechnology, 8, 52 (1997)). However, although finepatterns can be formed very precisely, these fabrication technologiesare not suitable for industrial applications because they need expensiveapparatuses and give low throughput.

It is also proposed to form nano-size projections not by theconventional lithographic technologies but by an etching process inwhich particles are used as an etching mask (JP-A 2006-512781(KOKAI)).According to this etching process, columnar structures having sizescorresponding to diameters of the particles can be formed on asubstrate. However, in the process, the selective etching ratio betweenthe particles and the substrate is too large to form projections havinghigh antireflection functions. It is also proposed to form projectionsby another etching process in which particles having a high selectiveetching ratio to the substrate is used as the mask. In the process,first the substrate is processed and then the particles are slimed whilethe etching gas is successively changed (JP-A 2005-331868 (KOKAI)).However, although projections can be almost obtained, this etchingprocess comprises complicated procedures since the etching gas must besuccessively changed. Further, bumps are formed in the projectionswhenever the etching gas is changed, so that the refractive index variesnot smoothly and, as a result, that the antireflection functions areimpaired. Furthermore, since all the particles serving as the mask arecompletely etched, the tips of the projections are so sharpened byside-etching that the refractive index changes steeply at the tips tolower the antireflection functions. This unfavorable effect is enhancedif the particles have more uneven sizes, and consequently theprojections are liable to have such uneven shapes that theantireflection functions are deteriorated and/or that the process marginis often narrowed to lower the productivity.

SUMMARY OF THE INVENTION

First, the present invention resides in a method for forming anantireflection structure in which plural projections are arranged on asubstrate, comprising the steps of:

-   -   forming a base layer on a substrate,    -   forming a particle-trap layer on said base layer,    -   forming, on said particle-trap layer, a multi-particle layer in        which particles having a mean particle size of 20 to 1000 nm are        arranged in one or more layers,    -   sinking the particles positioned at the bottom of said        multi-particle layer into the particle-trap layer,    -   removing the particles in said multi-particle layer except the        particles positioned at the bottom,    -   removing said particle-trap layer with holding the particles        positioned at the bottom on the base layer, and    -   performing an etching process in which said base layer is        subjected to reactive ion etching while the particles remaining        on said base layer are used as a mask to form projections, under        the conditions that the etching rate ratio of said base layer to        said particles is more than 1 but not more than 5 and that the        process is stopped before the particles disappear by etching.

Secondly, the present invention also resides in a method for forming anantireflection structure in which plural projections are arranged,comprising the steps of:

-   -   forming a base layer on a substrate,    -   forming a particle-trap layer on said base layer,    -   forming, on said particle-trap layer, a multi-particle layer in        which particles having a mean particle size of 20 to 1000 nm are        arranged in one or more layers,    -   sinking the particles positioned at the bottom of said        multi-particle layer into the particle-trap layer,    -   removing the particles in said multi-particle layer except the        particles positioned at the bottom,    -   removing said particle-trap layer with holding the particles        positioned at the bottom on the base layer,    -   performing an etching process in which said base layer is        subjected to reactive ion etching while the particles remaining        on said base layer are used as a mask to form projections, under        the conditions that the etching rate ratio of said base layer to        said particles is more than 1 but not more than 5 and that the        process is stopped before the particles disappear by etching, so        as to produce a master plate in which the plural projections are        arranged on the substrate,    -   using said master plate to produce a nanoimprint stamper having        a pattern reverse to the pattern formed on the master plate, and    -   performing a nanoimprinting process in which said nanoimprint        stamper is used to produce a replica of the pattern on the        master plate.

The present invention further resides in an antireflection structureformed by either of the above methods.

The present invention furthermore resides in a nanoimprint stamper forforming an antireflection structure, produced by the steps of:

-   -   forming a base layer on a substrate,    -   forming a particle-trap layer on said base layer,    -   forming, on said particle-trap layer, a multi-particle layer in        which particles having a mean particle size of 20 to 1000 nm are        arranged in one or more layers,    -   sinking the particles positioned at the bottom of said        multi-particle layer into the particle-trap layer,    -   removing the particles in said multi-particle layer except the        particles positioned at the bottom,    -   removing said particle-trap layer with holding the particles        positioned at the bottom on the base layer,    -   performing an etching process in which said base layer is        subjected to reactive ion etching while the particles remaining        on said base layer are used as a mask to form projections, under        the conditions that the etching rate ratio of said base layer to        said particles is more than 1 but not more than 5 and that the        process is stopped before the particles disappear by etching, so        as to produce a master plate in which the plural projections are        arranged on the substrate, and    -   using said master plate to produce a nanoimprint stamper having        a pattern reverse to the pattern formed on the master plate.

The present invention furthermore resides in a method for forming anantireflection structure in which plural projections are arranged,comprising the steps of:

-   -   forming a base layer on a substrate,    -   forming a particle-trap layer on said base layer,    -   forming, on said particle-trap layer, a multi-particle layer in        which particles having a mean particle size of 20 to 1000 nm are        arranged in one or more layers,    -   sinking the particles positioned at the bottom of said        multi-particle layer into the particle-trap layer,    -   removing the particles in said multi-particle layer except the        particles positioned at the bottom,    -   removing said particle-trap layer with holding the particles        positioned at the bottom on the base layer,    -   performing an etching process in which said base layer is        subjected to reactive ion etching while the particles remaining        on said base layer are used as a mask to form projections, under        the conditions that the etching rate ratio of said base layer to        said particles is more than 1 but not more than 5 and that the        process is stopped before the particles disappear by etching, so        as to produce a master plate in which the plural projections are        arranged on the substrate,    -   using said master plate to produce a replica having a pattern        reverse to the pattern formed on the master plate,    -   using said replica to produce a nanoimprint stamper having a        pattern reverse to the pattern formed on the replica, and    -   performing a nanoimprinting process in which said nanoimprint        stamper is used to produce a replica of the pattern on the        master plate.

The present invention furthermore resides in a nanoimprint stamper forforming an antireflection structure, produced by the steps of:

-   -   forming a base layer on a substrate,    -   forming a particle-trap layer on said base layer,    -   forming, on said particle-trap layer, a multi-particle layer in        which particles having a mean particle size of 20 to 1000 nm are        arranged in one or more layers,    -   sinking the particles positioned at the bottom of said        multi-particle layer into the particle-trap layer,    -   removing the particles in said multi-particle layer except the        particles positioned at the bottom,    -   removing said particle-trap layer with holding the particles        positioned at the bottom on the base layer,    -   performing an etching process in which said base layer is        subjected to reactive ion etching while the particles remaining        on said base layer are used as a mask to form projections, under        the conditions that the etching rate ratio of said base layer to        said particles is more than 1 but not more than 5 and that the        process is stopped before the particles disappear by etching, so        as to produce a master plate in which the plural projections are        arranged on the substrate,    -   using said master plate to produce a replica having a pattern        reverse to the pattern formed on the master plate, and    -   using said replica to produce a nanoimprint stamper having a        pattern reverse to the pattern formed on the replica.

The present invention in one embodiment provides a formation method bywhich an antireflection structure excellent in antireflection functionscan be produced in a large area at small cost. Further, the presentinvention in another embodiment provides an antireflection structureexcellent in antireflection functions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows sectional views schematically illustrating a method forforming an antireflection structure according to one embodiment of thepresent invention.

FIG. 2 shows sectional views schematically illustrating antireflectionstructures according to the present invention and the prior art.

FIG. 3 shows a scheme illustrating various parameters of theantireflection structure.

FIG. 4 shows graphs illustrating relations between the effectiverefractive index and the height of projections.

FIG. 5 shows sectional views schematically illustrating another methodfor forming an antireflection structure according to another embodimentof the present invention.

FIG. 6 shows a sectional view schematically illustrating a method forforming a stamper according to still another embodiment of the presentinvention.

FIG. 7 is an electron micrograph image showing a surface of anantireflection structure according to one embodiment of the presentinvention.

FIG. 8 shows sectional views schematically illustrating another methodfor forming an antireflection structure according to another embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below with theattached drawings referred to.

FIG. 1 shows sectional views schematically illustrating a method forforming an antireflection structure according to one embodiment of thepresent invention. Needless to say, FIG. 1 illustrates only a typicalexample for promoting understanding of the embodiment, and therefore itby no means restricts the antireflection structure formation method ofthe present invention.

On a substrate 101, a base layer 202 for forming an antireflectionstructure is provided (FIG. 1( a)). There is no particular restrictionon the material of the substrate, and hence any of inorganic materials,organic materials and organic and inorganic composite materials can beused. There is also no particular restriction on the size and thicknessof the substrate, and further the substrate may have any surface such asa flat surface or a curved surface. Examples of the substrate include aglass substrate, a quartz substrate and a Si substrate. In considerationof adhesion between the substrate and the base layer, the substrate maybe subjected to surface treatment before the base layer is formedthereon. Further, also before the base layer is formed, a materialhaving high durability against etching described later can beaccumulated on the substrate to form a stopper layer.

The base layer is made of such a material that the selective etchingratio thereof to the particles is in the range of more than 1 but notmore than 5, preferably in the range of more than 1 but not more than2.5 under the below-described etching conditions. There is no particularrestriction on the material of the base layer as long as the selectiveetching ratio thereof is in the above range, and hence any of inorganicmaterials, organic materials and organic and inorganic compositematerials can be used. When the base layer is processed by etching, theparticles serving as an etching mask are gradually slimed according asthe etching time elapses since the selective etching ratio of the baselayer to the particles is more than 1 but not more than 5. Thus,projections are formed on the substrate. If the selective etching ratiois too small, the particles cannot fully serve as a mask and hence theprojections formed from the base layer by etching have too low an aspectratio, for example, an aspect ratio of 1 or less, to obtain highantireflection effect. On the other hand, if the selective etching ratiois too large, the particles have such high durability against etchingthat tips of the projections in the pattern are difficult to be properlysharpened. In that case, if the etching time is prolonged to sharpen thetips, the projections undergo side etching to impair the antireflectioneffect. Accordingly, it is unfavorable. If the particles are made ofsilica materials, the base layer can be made of, for example, silicamaterials, silicone materials or silsesquioxane materials since theselective etching ratios of these materials to the particles are withinthe above range under the etching conditions for processing the baselayer. Among the above materials, silsesquioxane materials arepreferred. In the case where silsesquioxane materials are used, theprojections after formed are preferably subjected to heat treatment at atemperature of 400° C. to 1000° C. If the particles are mainly made ofpolystyrene resin, the base layer can be made of, for example, acrylicresin, phenol resin, polystyrene resin or polyimide resin. There is noparticular restriction on the method for forming the base layer, andhence thin-film formation methods generally known can be used. Examplesof the thin-film formation methods include wet processes such asspin-coating method, dip-coating method and squeegee method and dryprocesses such as vacuum-deposition method, sputtering method and CVDmethod. There is also no particular restriction on the thickness of thebase layer 202 as long as the thickness is larger than the height of theaimed antireflection structure, but it is normally 20 to 2500 nm,preferably 50 to 1000 nm.

Thereafter, a particle-trap layer 204 is formed on the base layer 202(FIG. 1( b)). The particle-trap layer combines the base layer with fineparticles, which serve as an etching mask when the base layer isprocessed by etching. If the particles are arranged in a mono-layer byconventional method such as spin-coating method without using theparticle-trap layer, they isometrically interact with each other throughintermolecular force and consequently are so unevenly dispersed thatthere are formed some areas where the particles are gathered densely andother areas where the particles are scarcely present. These areas causedefects of the resultant antireflection structure, and hence impair theantireflection functions. The particle-trap layer is generally made of amaterial comprising a polymer compound and satisfying the following fourconditions.

(1) The material can become fluid at least once when heated.

(2) The material has a glass transition point lower than any of theglass transition point, the melting point and the sintering temperatureof the particles.

(3) The material is not damaged by a particle-dispersion solution or bya washing solution. For example, the material is not dissolved or doesnot come off, and further the surface thereof is not roughened.

(4) The selective etching ratio of the material to the base layer andthe particles is more than 1 under the conditions where theparticle-trap layer is removed by etching.

As the polymer compound, either an organic polymer compound or aninorganic polymer compound can be used. There is no particularrestriction on the molecular weight of the polymer compound. Thematerial of the particle-trap layer may be a composition containingadditives such as a plasticizer, if necessary. For example, an organicthermo-plastic polymer compound mixed with desired additives isgenerally used as the composition, and it is also possible to use aB-staged thermo-setting resin. Further, in the case where theantireflection structure is required to be formed in a particular areasuch as a patterned area on the substrate, a patterned particle-traplayer can be photo-lithographically formed from a resin compositioncontaining photosensitive substance. There is no particular restrictionon the method for forming the particle-trap layer, and hence any methodcan be used. Normally, the particle-trap layer is formed by coating thesubstrate with a solution or suspension containing materials of theparticle-trap layer. There is no particular restriction on the coatingmethod, and hence any of known coating methods such as spin-coatingmethod, dip-coating method and squeegee method can be used. Among them,the spin-coating method is preferred since it can control nano-scalethickness to form a thin membrane. In the case where the particle-traplayer is to be formed on a large substrate, the dip-coating method isalso preferred.

The thickness of the particle-trap layer is preferably approx. ⅓ of themean particle size of the particles. As described later, the particles203 placed on the particle-trap layer are sunk in the particle-traplayer. Thereafter, the excess particles not sunk in the particle-traplayer 204 are removed by washing. In this step, since the particles sunkin the particle-trap layer are kept in contact with the particle-traplayer in large areas to have sufficient adhesion, they can escape fromremoving to remain. Accordingly, in the case where it is necessary towash away the excess particles, the particles to remain are preferablysunk in the particle-trap layer to a certain depth. The thickness of theparticle-trap layer, therefore, depends upon the diameters of theparticles, but is generally 6 to 350 nm, preferably 20 to 120 nm.

On the particle-trap layer, particles 203 are arranged to form amulti-particle layer 206 (FIG. 1( c)). As described above, the materialof the particles must have a selective etching ratio to the base layerin a particular range under the etching conditions for processing thebase layer. The particles have a mean particle size of 20 to 1000 nm,preferably 50 to 350 nm. If the mean particle size is 20 nm or less,projections having sufficient height such as 100 nm or more cannot beformed and accordingly the antireflection effect on visible light,particularly, in the near infrared region is lowered. Accordingly, it isunfavorable. On the other hand, if the mean particle size is 1000 nm ormore, the intervals among the formed projections are 1000 nm or more andaccordingly light in the visible region is diffracted to impair theantireflection functions. It is hence unfavorable, too. In the presentinvention, the mean particle size can be determined on the basis ofprojected cross-section areas of the particles observed by scanningelectron microscopy, and it corresponds to an average diameter of theparticles on the assumption that they are spherical.

The particles are preferably in the shape of almost perfect sphere. Thisis because the particles in the shape of almost perfect sphere can bedensely arranged when aggregated and, as a result, projections can bedensely formed. The CV (coefficient of variant) value of the particlesizes is preferably 15% or less, more preferably 10% or less. If the CVvalue is more than 15%, it is difficult to arrange the particles denselybecause the particles have very different sizes. As a result, in thatcase, the density of the projections is lowered and the formedprojections have very different shapes, and accordingly theantireflection effect is often impaired.

The multi-particle layer can be easily formed by coating aparticle-dispersion solution on the particle-trap layer and then byevaporating the dispersion medium. There is no particular restriction onthe method for coating the particle-dispersion solution, and hence anyof known coating methods such as spin-coating method, dip-coating methodand squeegee method can be used. The dispersion medium is evaporated todry preferably at a temperature lower than the glass transition point ofthe polymer compound used in the particle-trap layer. There is noparticular restriction on the drying method. For example, if thespin-coating method is employed, the sample may be kept rotating untildried or otherwise may be rotated for a few seconds to spread thedispersion medium and then dried naturally, by heating or by blowing ofnitrogen gas. Also in the case where the dip-coating method or thesqueegee method is employed, the sample can be dried naturally, byheating or by blowing of nitrogen gas. If the coating procedure or thedrying procedure is performed at a temperature not lower than the glasstransition point of the particle-trap layer, the material of theparticle-trap layer often partly dissolves into the particle-dispersionsolution and consequently remains among the particles after thedispersion medium is dried. It should be noted that the material thusremaining is liable to prevent the particles from being removed bywashing.

While the particle-dispersion solution spread on the trap layer is beingdried, the particles are gradually gathered densely by the actions ofthe surface tension of the dispersion solution and of the intermolecularforce among the particles. As a result, a multi-particle layer in whichthe particles are densely arranged is formed. In the case where theabove procedures are carried out at a temperature lower than the glasstransition point, the multi-particle layer is not sufficiently fixed onthe particle-trap layer.

In order to improve wettability of the particle-dispersion solution whencoated on the particle-trap layer, surfactants may be added into thedispersion solution and/or the trap layer may be beforehand subjected tosurface treatment. For example, in the case where the dispersion mediumis water and the tap layer is made of hydrophobic resin, the dispersionmedium is often so repelled that the multi-particle layer cannot beevenly formed because they are very different in surface energy.However, if the surface of the trap layer is made to be hydrophilic byknown treatment such as UV washing or oxygen plasma treatment, themulti-particle layer can be evenly formed on the whole surface of thesubstrate.

After the multi-particle layer 206 is formed, the substrate is heated toa temperature not lower than the glass transition point of theparticle-trap layer 204 so that the particles 203 can be sunk in theparticle-trap layer 204 (FIG. 1( d)). There is no particular restrictionon the method for heating, and hence known methods are usable. Forexample, the substrate may be heated on a hot-plate or in a heatingfurnace. It is also possible to heat only a part of the substrate bylaser exposure. As shown in FIG. 1( d), the sample is preferably heatedfrom the side opposite to the multi-particle layer, namely, from thesubstrate side. If the sample is heated from the particle layer side,the particle-trap layer is often so unevenly heated that the particlesare sunk unevenly to form defects in the case where the particles in themulti-particle layer are unevenly layered, in other words, where themulti-particle layer has uneven thickness.

When heated, the particle-trap layer becomes fluid and rises to soakinto voids of the multi-particle layer. How high the trap layer rises isdetermined by the original volume of the trap layer and by the volume ofthe voids among the particles. The multi-particle layer is thus sunk andfixed in the trap layer to the depth depending upon the thickness of thetrap layer. In many cases, the heating time for sinking the particles203 in the trap layer 204 is approx. 1 minute. During this procedure,the multi-particle layer may be pressed downward so that the particlescan be easily sunk. Further, after the particles are sunk in the traplayer, the trap layer may be heated above the curing temperature toharden thermally or otherwise the trap layer beforehand incorporatedwith a photo-polymerization initiator may be exposed to UV light toharden photo-chemically so as to increase adhesion between the particlesand the trap layer.

Thereafter, the substrate is washed with a washing solution and therebythe particles in the multi-particle layer 206 except those sunk in thetrap layer are removed to form a mono-particle layer 205 (FIG. 1( e)).The substrate is preferably washed at a temperature lower than the glasstransition point of the particle-trap layer. If it is not lower, thetrap layer is so softened that the particles caught in the trap layermay come off or that the particles washed off may reattach. There is noparticular restriction on the washing solution, and hence water, variousorganic solvents and mixtures thereof can be used. Examples of theorganic solvents include alcohols such as methyl alcohol, ethyl alcoholand isopropyl alcohol. The washing solution may contain additives suchas surfactants.

The substrate on which the mono-particle layer is thus attached is thensubjected to dry etching to remove the particle-trap layer (FIG. 1( f)).The etching procedure is performed under the condition that theselective etching ratios of the base layer and the particles to theparticle-trap layer are less than 1, preferably less than 0.5, andthereby only the particle-trap layer can be selectively removed whilethe base layer and the particles are scarcely etched. In this step, themono-particle layer, namely the particles positioned at the bottom isheld. The etching gas is preferably selected to be effective in etchingthe trap layer, and hence is, for example, oxygen gas in the case wherethe trap layer is made of an organic polymer mainly comprising carbonatoms. On the other hand, if the trap layer is made of a siliconatom-containing material such as silicone resin, an etching gas mainlycomprising fluorine-type gas can be used.

Finally, the substrate on which the mono-particle layer is thus formedis then subjected to dry etching to form a pattern of projections (FIGS.1( g) and (h)). The etching procedure is performed under the conditionthat the selective etching ratio of the base layer to the particles iswithin the aforementioned range, and thereby the particles are graduallyetched to reduce the particle sizes according as the base layer 202proceeds to be etched. As a result, the base layer is etched to form apattern of projections 102 having high aspect ratios, such as aspectratios of 1 to 5. The etching process is stopped so that the particles203 a having smaller diameters than those before etched can be left onthe tips of the projections, to obtain an antireflection structurehaving high antireflection functions. Since the particles are left onthe tips, the projections of the pattern have round tips as shown inFIG. 2( a). By way of example, the shapes of the projections are shownin FIG. 7. If the etching process is continued until the particles onthe tips disappear, the resultant projections have sharp tips as shownin FIG. 2( b).

The particles having a small mean particle size are thus left on thetips, and they give the effect explained below with the attacheddrawings referred to. FIG. 3 schematically illustrates a projectionformed on the base having a refractive index N1. In the figure, N0stands for the external refractive index. The refractive index of theprojection at the height H depends upon an area ratio of its crosssection parallel to the substrate surface. This is represented by theformula (1):Neff=N0·(1−S1)+N1·S1  (1)in which Neff is the refractive index and S1 is the area ratio at theheight H. Since the projection has a conical shape, the area ratio S1and the radius R(H) of the cross section at the height H satisfy therelation represented by the formula (2):S1=k·(R(H))²  (2)in which k is a proportionality constant. Here, a projection-shapefunction A(H) is defined by the outline curve of its vertical sectionperpendicular to the substrate. In the case where the tips aresharpened, the projection-shape function A(H) is in proportion to theheight H and hence expressed by the formula (3):A(H)=C·H  (3)in which C is a constant corresponding to the inclination of theprojection outline. The refractive index of the projection at the heightH is calculated from the formulas (1) to (3) to obtain a straight linesuch as a straight line (a) in FIG. 4. On the other hand, in the casewhere the tips are rounded as those in the present invention, theprojection-shape function A(H) can be fitted into a quadratic functionand hence expressed by the formula (4):A(H)=C·H ²  (4)in which C is a constant. The refractive index of the projection at theheight H is calculated from the formulas (1), (2) and (4), to obtain acurve such as a curved line (b) in FIG. 4. In comparison between thelines (a) and (b) in FIG. 4, it is clear that the refractive index ofthe projection formed in the present invention varies smoothly from thesubstrate side to the outside and that there is no range where therefractive index varies steeply. On the other hand, it is also clearthat, if the etching process is continued not to leave the particles 203a and thereby to sharpen the tips of the projections, there is a rangewhere the refractive index varies steeply. Accordingly, the projectionsof the present invention, in which the refractive index varies smoothly,give high antireflection effect.

The shapes of the particles left on the tips of the projections dependupon size distribution of the particles serving as the etching mask.Since each particle reduces the same volume by etching, particles havingsmall diameters before etching disappear rapidly. Accordingly, the sizedistribution of the particles serving as the etching mask is controlledso as to obtain an optimal antireflection structure. The presentinventors have studied and found that, if the remaining particles 203 ahave a relative volume of 25% or less based on the volume of theparticles 203 before the etching process, the formed projection has aquadratic outline and hence the refractive index varies favorably.Therefore, in order that the refractive index may vary favorably, theremaining particles have a relative volume of preferably more than 0%but not more than 25%, further preferably 5 to 15%, based on the volumeof those before the etching process. The etching process is preferablyso performed that 80% or more number of particles arranged in themono-particle layer 205 on the substrate may reduce the volume into 25%or less based on the volume of the particles 203 before the etchingprocess, and thereby 70% or more, 80% or more, or 90% or more number ofthe particles are preferably left under the condition that the CV valueis in the range of 15% or more, in the range of 5 to 15%, or in therange of less than 5%, respectively. Thus, high antireflection effectcan be obtained in the present invention. Here, the “CV value” iscalculated as the standard deviation of the particle sizes divided bythe mean particle size, and is widely used as a value indicating theparticle size distribution.

Further, if the etching process of the base layer is continued until allthe particles completely disappear, evenness of the pattern is impaired.Since small particles disappear rapidly, the areas masked with the smallparticles are bared early and hence are over-etched to cause patterncollapse if the etching process is continued until large particles alsodisappear. As a result, the antireflection effect is seriously impaired.The etching process, therefore, is stopped before the particlesdisappear, and thereby the process margin can be widened.

It can be confirmed by observation with scanning electron microscopywhether the particles are left on the tips of the projections. Theprojections formed from the base layer and the particles left on thetips thereof can be clearly distinguished if the section of the formedantireflection structure is observed. In order that the etching processcan be stopped before the particles disappear, the etching time isvariously changed and the section is observed to obtain a calibrationcurve by which it can be determined when the particles disappear andhence when the etching process should be stopped.

In the antireflection structure obtained by the method of the presentinvention, the projections are generally arranged with imperfectperiodicity. The structure often comprises plural domains adjoiningrandomly, and each domain has a size of a few micrometers to a fewmillimeters and comprises the projections regularly arranged in atriangle grating or in a square grating. If incident light comes at adeep angle into the projections arranged with perfect periodicity, thelight is diffracted to cause troubles in some applications such asdisplays. In contrast, the structure of the present invention can avoidthe diffraction of light without impairing the antireflection functions.

In the following description, the antireflection structure formationmethod according to another embodiment of the present invention isexplained with FIG. 5 referred to. Needless to say, FIG. 5 illustratesonly a typical example for promoting understanding of the embodiment,and therefore it by no means restricts the method of the presentinvention.

The steps shown in FIGS. 5( a) to 5(h) can be carried out in the samemanner as described above, to form projections. The substrate 101 onwhich the projections 602 are thus formed is used not as theantireflection structure in itself but as a master plate 607 for formingthe antireflection structure (FIG. 5(h)). By use of the master plate607, a nanoimprint stamper 608 having a reverse pattern is produced. Thenanoimprint stamper can be formed by any of the known methods such as anelectrodeposition process. For example, the nanoimprint stamper can beproduced by an electrodeposition process in the following manner. First,a thin electro-conductive membrane made of a metal such as Ni (notshown) is formed on the master plate, and then the plate is subjected toan electro-chemical reaction in an electrodeposition bath to form anelectrodeposition membrane 608 a having a thickness of dozens ofmicrometers to several millimeters (FIG. 5( i)). After the master plateis taken out of the electrodeposition bath, the electrodepositionmembrane is peeled off to obtain a nanoimprint stamper 608 (FIG. 5( j)).When the nanoimprint stamper is peeled from the master plate, theprojection pattern 607 on the plate is often partly released togetherand attached on the stamper. In that case, the surface of the stamper iswashed with a washing solution or cleaned by dry processes such asetching to obtain a clean surface. Further, the back surface of thestamper may be polished to be flat so that pressure can be evenlyapplied in the nanoimprinting process performed later.

When peeled off, the stamper may be wound up in a roll by means of adrum or a belt-type support, as shown in FIG. 6, to form a stamper roll.The stamper roll is preferably used in a roll imprinting process or in aroll-to-roll imprinting process, and hence is particularly advantageousin the case where the antireflection structure is formed bynanoimprinting in a large area such as a display substrate or a guideplate of LCD or in the case where high throughput is required, forexample, in producing an antireflection film. Needless to say, thestamper may be released from the master plate and thereafter attachedonto a drum or a belt-type support. The nanoimprint stamper formed byelectrodeposition generally keeps sufficient strength even in ahigh-pressure imprinting process, and hence is particularly suitable foran imprinting process at room temperature or at an elevated temperature.The pattern of the master plate can be transferred onto thermosettingresin or photo-curable resin, to prepare a nanoimprint stamper having apattern reverse to the pattern of the master plate. If the master plateis directly used as a stamper to perform imprinting, mechanical strengthof the master plate may be enhanced to reduce damages suffered by themaster pattern in the imprinting process. For example, a silsesquioxanematerial is used as the material of the base layer, from which theantireflection structure is formed. The formed antireflection structureis then subjected to high-temperature heat treatment in the range of400° C. to 1000° C., and thereby the silsesquioxane material isconverted into SiO_(x). As a result, the master plate has higherstrength than before the heat treatment, so as to reduce damagessuffered by the master plate in the imprinting process.

Thereafter, by use of the nanoimprint stamper 608, a second base layer610 formed on a second substrate 609 is subjected to nanoimprinting toproduce a replica 611 having almost the same pattern as the masterpattern. In order that the replica can be easily peeled from the stamperafter nanoimprinting, the stamper and/or the base layer may bebeforehand coated with a releasing agent comprising substances havinglow surface energies, for example, fluorine-containing compounds. Thesecond substrate and the second base layer may be made of the samematerial. This means that the second substrate serving as a support maybe omitted if the second base layer has sufficient strength. There is noparticular restriction on the material of the second substrate as longas it has enough strength not to be destroyed in the imprinting process.Accordingly, any of inorganic materials, organic materials and organicand inorganic composite materials can be used. There is also noparticular restriction on the size and thickness thereof. However, thesubstrate preferably has a flat surface so that pressure can be evenlyapplied in the imprinting process. Examples of the second substrateinclude a Si substrate, a quartz substrate, a glass substrate and aplastic substrate of resins such as PET resin and polycarbonate resin.Further, in consideration of adhesion between the second substrate andthe second base layer provided thereon, the second substrate may besubjected to proper surface treatment.

There is no particular restriction on the material of the second baselayer 610 as long as a pattern reverse to the pattern of the stamper canbe transferred by nonoimprinting. For some applications, such as opticalfilms, required to have high transparency in the visible region,materials having little absorbance in the visible region are preferablyused.

In the case where a thermal imprinting process is performed, the secondsubstrate before pressed is preferably heated at a temperature not lowerthan the glass transition point of the second base layer so that thesecond base layer becomes fluid. Examples of the material for the secondbase layer in the thermal imprinting process include thermoplastic orthermosetting resins such as acrylic resins, polystyrene resins,polyimide resins, polyester resins, epoxy resins, polycarbonate resins,melanin resins, cellulose resins, silicone resins, and mixtures thereof.Further, additives such as plasticizers and releasing agents can beincorporated.

In the case where a photo-imprinting process is performed, the secondbase layer during or after imprinting is irradiated with radiations suchas visible light, UV light and electron beams to cause polymerizationreaction and thereby to form the replica. Examples of the material forthe second base layer in the photo-imprinting process include acrylicresins, polyester resins, epoxy resins, polystyrene resins, polyurethaneresins, phenol resins, cellulose resins, silicone resins andsilsesquioxane materials, provided that they contain photo-polymerizablefunctional groups. Further, it is also possible to use a resincomposition comprising a binder of the above resins having nophoto-polymerizable functional group, a monomer or oligomer havingphoto-polymerizable functional groups, and additives such as aphoto-polymerizaton initiator, a polymerizaton inhibitor and a diluent.The resin composition may further contain a releasing agent so that thestamper after imprinting can be easily peeled off. There is noparticular restriction on the method for forming the second base layer,and hence thin-film formation methods generally known can be used.Examples of the thin-film formation methods include wet processes suchas spin-coating method, dip-coating method and squeegee method and dryprocesses such as vacuum-deposition method, sputtering method and CVDmethod. There is also no particular restriction on the thickness of thesecond base layer as long as the thickness is larger than the height ofthe aimed antireflection structure. The aforementioned nanoimprintstamper and the second base layer are preferably made of silsesquioxanematerials. After formed from the silsesquioxane materials, the structureis preferably subjected to heat treatment at a temperature of 400° C. to1000° C. so as to obtain excellent strength.

Furthermore, a replica which is produced by the above-mentioned methodcan be used to produce a antireflection structure. First, the replica612 is produced by the method which is similar to the method producingthe nanoimprint stamper (FIG. 8 (a) to (j)). Then, by use of the replica612, a nanoimprint stamper 608 having a pattern reverse to the patternformed on the replica is produced, for example, by an electrodepositionprocess (FIG. 8 (k)). Thereafter, by use of the nanoimprint stamper 608,a second base layer 610 formed on a second substrate 609 is subjected tonanoimprinting to produce a pattern 611 having almost the same structureas the replica 612 (FIG. 8 (l) to (m)). The structure 611 has a patternreverse to the pattern formed on the master plate. Thus, a refractiveindex of the structure 611 varies smoothly, and gives highantireflection effect.

The antireflection structure of the present invention can be applied forforming displaying screens of various instruments such as cellularphones, for producing surfaces of LCD guide plates and for manufacturingilluminating surfaces of lighting devices. Accordingly, the presentinvention can be favorably used for producing, for example, cellularphones, displays, lighting devices, watches, PCs, and music players.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

EXAMPLES

The present invention is further explained below by the followingexamples, which by no means restrict the present invention.

Example 1

As a substrate, a 4-inch quartz substrate (synthesized quartz glass AQ™,manufactured by Asahi Glass Co., Ltd.) was prepared. On the substrate,spin-on glass material (hereinafter, often referred to as “SOGmaterial”) (OCD-T7™, manufactured by Tokyo Ohka Kogyo Co., Ltd.), as amaterial for the base layer from which the antireflection structure wasto be formed, was spin-coated at 2000 rpm for 40 seconds, and then bakedon a hot-plate at 110° C. for 90 seconds. The substrate was annealed ina N₂ oven at 300° C. for 35 minutes. The thickness of the SOG membranethus formed was observed by scanning electron microscope (hereinafter,often referred to as “SEM”), and found to be 450 nm. Further, a resistfor half-micron pattern (THMR-ip3250™, manufactured by Tokyo Ohka KogyoCo., Ltd.), as a material for the particle-trap layer, was spin-coatedat 2000 rpm for 35 seconds, and then baked on a hot-plate at 110° C. for90 seconds. The thickness of the particle-trap layer thus formed wasobserved and found to be 55 nm, which was suitable for catching only amono-layer of particles having a mean particle size of 200 nm.Thereafter, the surface of the particle-trap layer was made hydrophilicby mean of parallel plate etching system (DEM-451™, manufactured byCanon ANELVA Corporation) under the conditions that: the etching gas wasO₂, the flowing amount was 30 sccm, the pressure was 0.1 Torr, RF powerwas 100 W and the etching time was 5 seconds. Further, colloidal silicadispersed in water (PL-13™, manufactured by Fuso Chemical Co., Ltd.;mean size of silica particles: 200 nm; CV value: 10%; concentration ofsilica particles: 30 wt. %) was spin-coated at 1000 rpm for 60 secondsto form a multi-particle layer in which the particles were arranged in afew layers. The substrate was then baked on a hot-plate at 210° C. for30 minutes, so that the particles only at the bottom of themulti-particle layer were attached onto the substrate. The obtainedsubstrate was immersed in water and subjected to ultrasonic washing for10 minutes, and then the water was drained out. After pure water forwashing was newly introduced, ultrasonic washing was carried out againfor 1 minute to remove excess particles, which were not attached on thesubstrate. The section of the obtained substrate was observed by SEM,and thereby it was confirmed that silica particles partly sunk in thetrap layer were arranged in a mono-layer.

Successively, the particle-trap layer was removed by dry-etching underthe conditions that: the etching gas was O₂, the flowing amount was 30sccm, the pressure was 0.01 Torr, RF power was 100 W and the etchingtime was 1 minute. The section of the obtained substrate was observed bySEM, and thereby it was confirmed that the trap layer filling voidsamong the particles on the substrate was removed. The SOG base layer wasfurther processed by dry-etching under the conditions that: the etchinggas was CF₄, the flowing amount was 30 sccm, the pressure was 0.01 Torr,the electric power was 100 W and the etching time was 8 minutes. Theobtained substrate was observed in detail by SEM. As a result, it wasconfirmed that the silica particles were slimed by etching and therebythat the SOG material was processed to form projections. It was alsoobserved that the silica particles having served as an etching mask wereleft at the tips of the projections. The remaining particles had arelative volume of approx. 20% based on the volume of those beforeprocessed, and they were left on the tips of approx. 90% of all theprojections. Because of the remaining particles, the obtainedprojections had round tips. The average interval among the projectionsand the average height of the projections were 210 nm and 350 nm,respectively. Further, the shape of the formed projection was measuredto obtain the projection-shape function A(H) fitted into a quadraticfunction (5):A(H)=−0.0126H ²+350  (5)which indicated that the refractive index was moderately distributed.

The transmission and reflection spectra of the surface on which theantireflection structure was thus formed were measured. As a result, thetransmittances at the wavelengths of 470 nm, 550 nm and 630 nm werefound to be 99.5%, 99.7% and 99.7%, respectively. Accordingly, it wasfound that high transmittances were obtained in the whole visibleregion. On the other hand, the reflectances at the wavelengths of 470nm, 550 nm and 630 nm were found to be 0.4%, 0.3% and 0.3%,respectively. Accordingly, it was found that the reflectances werelargely reduced as compared with those (4.1%, 3.9% and 4.0%,respectively) before the antireflection structure was formed.

Example 2

As a substrate, a 6-inch Si substrate (manufactured by SUMCOCorporation) was prepared. The procedure of Example 1 was then repeatedto form an antireflection structure comprising 350 nm-high SOGprojections arranged at the interval of 210 nm on the 6-inch Sisubstrate. The obtained substrate was washed with water for 10 minutes,and dried on a hot-plate at 100° C. The silica particles on the tips ofthe projections were reacted with the SOG interface during the etchingprocess, and thereby fixed so firmly that they did not come from thetips in washing.

The substrate after washed was then used as a master plate to form ananoimprint stamper in the following manner. First, anelectro-conductive membrane of Ni was formed on the substrate by asputtering process. The sputtering process was carried out in a chamberwhich was beforehand evacuated to 8×10⁻³ Pa and then into which argongas was introduced until the inner pressure reached 1 Pa. While purenickel metal was used as a target, DC power of 400 W was applied for 65seconds to form an approx. 50 nm-thick Ni electro-conductive membrane.Thereafter, the master plate on which the electro-conductive membranewas thus formed was then immersed in a plating solution of nickelsulfamate (NS-160™, manufactured by Showa Chemical Industry CO., LTD.),and subjected to a Ni electrodeposition process for 90 minutes to forman approx. 300 μm-thick electrodeposition membrane. The conditions ofelectrodeposition bath were as follows:

Nickel sulfamate: 600 g/L,

Boric acid: 40 g/L,

Surface active agent (sodium lauryl sulfate): 0.15 g/L,

pH: 4.0, and

Current density: 20 A/dm².

The electrodeposition membrane and the electro-conductive membrane werepeeled from the master plate, to obtain a nanoimprint stamper. The SOGon the master plate was partly attached on the surface of the obtainedstamper, and hence the surface was cleaned to remove the residue byetching treatment under the conditions that: the etching gas was CF₄,the flowing amount was 30 sccm, the pressure was 0.06 Torr and RF powerwas 100 W. The surface of the stamper was observed by SEM, and it wasconfirmed that a hole pattern reverse to the projection pattern on themaster plate was formed on the whole surface of the stamper. Thereafter,burrs of the stamper were removed by punching with an edged metal punchto obtain a desirably shaped nanoimprint stamper.

The obtained nanoimprint stamper was treated with a releasing agent fornanoimprint (Durasurf HD-1100™, available from Daikin Chemicals SalesCo., Ltd.). The releasing agent was spin-coated on the stamper at 3000rpm for 20 seconds, and then baked on a hot-plate at 60° C. for 1 hour.

Successively, a nanoimprinting process was performed by use of theobtained stamper. First, a glass substrate of 100 mm×100 mm wasprepared. Then, polymethyl methacrylate (PMMA, Mw: 20000) was dissolvedin polyethylene glycol monomethyl ether acetate (PGMEA) in 30 wt. %, andthe obtained solution was spin-coated on the substrate at 2000 rpm for40 seconds to form a 2.5 μm-thick PMMA membrane. The membrane wassubjected to thermal imprint using the Ni stamper. While the substratewas heated at 150° C., the stamper was imprinted under 0.5 MPa for 2minutes. After the substrate was cooled to room temperature, the stamperwas released. The structure thus formed on the PMMA surface was observedby SEM, and it was found that the formed antireflection structure hadalmost the same pattern as the master plate had.

The transmission and reflection spectra of the surface on which theantireflection structure was thus formed were measured. As a result, thetransmittances at the wavelengths of 470 nm, 550 nm and 630 nm werefound to be 99.5%, 99.7% and 99.7%, respectively. Accordingly, it wasfound that high transmittances were obtained in the whole visibleregion. On the other hand, the reflectances at the wavelengths of 470nm, 550 nm and 630 nm were found to be 0.4%, 0.3% and 0.3%,respectively. Accordingly, it was found that the reflectances werelargely reduced as compared with those (4.1%, 3.9% and 4.0%,respectively) before the antireflection structure was formed.

Example 3

As a substrate, a 6-inch Si substrate (manufactured by SUMCOCorporation) was prepared. After a resist for half-micron pattern(THMR-ip3250™, manufactured by Tokyo Ohka Kogyo Co., Ltd.) was dilutedwith ethyl lactate to 30%, the obtained solution was spin-coated on theSi substrate at 2000 rpm for 40 seconds and then baked on a hot-plate at100° C. for 90 seconds. The substrate was further annealed in a N₂ ovenat 250° C. for 1 hour to harden the resist and thereby to form a resistlayer (base layer). The thickness of the resist layer was measured witha stylus profilometer (Tallystep™, manufactured by Taylor Hobson K.K.)and found to be 400 nm. Successively, a 55 nm-thick PMMA membrane wasformed thereon as the particle-trap layer. The surface of theparticle-trap layer was then made hydrophilic by mean of parallel plateetching system (DEM-451™, manufactured by Canon ANELVA Corporation)under the conditions that: the etching gas was O₂, the flowing amountwas 30 sccm, the pressure was 0.1 Torr, RF power was 100 W and theetching time was 5 seconds. Further, heat-resistant polystyreneparticles dispersed in water (manufactured by JSR Corporation; meanparticle size: 200 nm; CV value: 5%; concentration of particles: 30 wt.%) was spin-coated at 1000 rpm for 60 seconds to form a multi-particlelayer in which the particles were arranged in a few layers. The Sisubstrate was then baked on a hot-plate at 130° C. for 5 minutes, sothat the particles only at the bottom of the multi-particle layer wereattached onto the substrate. The obtained substrate was immersed inwater and subjected to ultrasonic washing for 10 minutes, and then thewater was drained out. After pure water for washing was newlyintroduced, ultrasonic washing was carried out again for 1 minute toremove excess particles, which were not attached on the substrate. Thesection of the obtained substrate was observed by SEM, and thereby itwas confirmed that polystyrene particles partly sunk in the trap layerwere arranged in a mono-layer.

Successively, the particle-trap layer was removed by dry-etching underthe conditions that: the etching gas was O₂, the flowing amount was 30sccm, the pressure was 0.01 Torr, the electric power was 100 W and theetching time was 25 seconds. The section of the obtained substrate wasobserved by SEM, and thereby it was confirmed that the PMMA fillingvoids among the particles on the substrate was removed. The resist layerwas further processed by dry-etching under the conditions that: theetching gas was O₂, the flowing amount was 30 sccm, the pressure was0.01 Torr, RF power was 100 W and the etching time was 2.5 minutes. Theobtained substrate was observed in detail by SEM. As a result, it wasconfirmed that the polystyrene particles were slimed by etching andthereby that the resist layer was processed to form projections. It wasalso observed that the polystyrene particles having served as an etchingmask were left at the tips of the projections. The remaining particleshad a relative volume of approx. 15% based on the volume of those beforeprocessed, and they were left on the tips of approx. 95% of all theprojections. Because of the remaining particles, the obtainedprojections had round tips. The average interval among the projections,the average height of the projections and the aspect ratio of theprojections were 200 nm, 380 nm and 1.9, respectively.

The substrate thus obtained was then used as a master plate to form ananoimprint stamper by a Ni electrodeposition process, and a PMMAmembrane was subjected to a nanoimprining process by use of thenanoimprint stamper. The Ni electrodeposition process and thenanoimprining process were carried out in the same manner as in Example2. The structure formed on the PMMA surface was observed by SEM, and itwas confirmed that the formed antireflection structure had almost thesame pattern as the master plate had.

The transmission and reflection spectra of the surface on which theantireflection structure was thus formed were measured. As a result, thetransmittances at the wavelengths of 470 nm, 550 nm and 630 nm werefound to be 99.6%, 99.7% and 99.6%, respectively. Accordingly, it wasfound that high transmittances were obtained in the whole visibleregion. On the other hand, the reflectances at the wavelengths of 470nm, 550 nm and 630 nm were found to be 0.2%, 0.3% and 0.3%,respectively. Accordingly, it was found that the reflectances werelargely reduced as compared with those (4.1%, 3.9% and 40.0%,respectively) before the antireflection structure was formed.

Example 4

As a substrate, a 4-inch quartz substrate (synthesized quartz glass AQ™,manufactured by Asahi Glass Co., Ltd.) was prepared. On the substrate,SOG material (OCD-T12™, manufactured by Tokyo Ohka Kogyo Co., Ltd.), asa material for the base layer from which the antireflection structurewas to be formed, was spin-coated at 2000 rpm for 40 seconds, and thenbaked on a hot-plate at 110° C. for 90 seconds. The substrate wasannealed in a N₂ oven at 300° C. for 35 minutes. The thickness of theSOG membrane thus formed was observed by SEM, and found to be 480 nm.Further, a resist for half-micron pattern (THMR-ip3250™, manufactured byTokyo Ohka Kogyo Co., Ltd.), as a material for the particle-trap layer,was spin-coated at 2000 rpm for 35 seconds, and then baked on ahot-plate at 110° C. for 90 seconds. The thickness of the particle-traplayer thus formed was observed and found to be 55 nm, which was suitablefor catching only a mono-layer of particles having a mean particle sizeof 200 nm. Thereafter, the surface of the particle-trap layer was madehydrophilic by mean of parallel plate etching system (DEM-451™,manufactured by Canon ANELVA Corporation) under the conditions that: theetching gas was O₂, the flowing amount was 30 sccm, the pressure was 0.1Torr, RF power was 100 W and the etching time was 5 seconds. Further,colloidal silica dispersed in water (PL-13™, manufactured by FusoChemical Co., Ltd.; mean size of silica particles: 200 nm; CV value:10%; concentration of silica particles: 30 wt. %) was spin-coated at1000 rpm for 60 seconds to form a multi-particle layer in which theparticles were arranged in a few layers. The substrate was then baked ona hot-plate at 210° C. for 30 minutes, so that the particles only at thebottom of the multi-particle layer were attached onto the substrate. Theobtained substrate was immersed in water and subjected to ultrasonicwashing for 10 minutes, and then the water was drained out. After purewater for washing was newly introduced, ultrasonic washing was carriedout again for 1 minute to remove excess particles, which were notattached on the substrate. The section of the obtained substrate wasobserved by SEM, and thereby it was confirmed that silica particlespartly sunk in the trap layer were arranged in a mono-layer.

Successively, the particle-trap layer was removed by dry-etching underthe conditions that: the etching gas was O₂, the flowing amount was 30sccm, the pressure was 0.01 Torr, RF power was 100 W and the etchingtime was 1 minute. The section of the obtained substrate was observed bySEM, and thereby it was confirmed that the trap layer filling voidsamong the particles on the substrate was removed. The SOG base layer wasfurther processed by dry-etching under the conditions that: the etchinggas was CF₄ gas, the flowing amount was 30 sccm, the pressure was 0.01Torr, RF power was 100 W and the etching time was 9 minutes. Theobtained substrate was observed in detail by SEM. As a result, it wasconfirmed that the silica particles were slimed by etching and therebythat the SOG base layer was processed to form projections. It was alsoobserved that the silica particles having served as an etching mask wereleft at the tips of the projections. The remaining particles had arelative volume of approx. 15% based on the volume of those beforeprocessed, and they were left on the tips of approx. 85% of all theprojections. Because of the remaining particles, the obtainedprojections had round tips. The average interval among the projections,the average height of the projections and the aspect ratio of theprojections were 210 nm, 310 nm and 1.7, respectively. The obtainedsubstrate was annealed in air at 800° C. for 30 minutes to convert SOGinto SiO_(x), and thereby the mechanical strength was enhanced. Sincethe SOG material had a small thermal shrinkage ratio, the pattern shapewas scarcely changed after the anneal treatment.

Thereafter, a releasing agent for nanoimprint (Durasurf HD-1100™,available from Daikin Chemicals Sales Co., Ltd.) was spin-coated on thesubstrate at 3000 rpm for 20 seconds, and then baked on a hot-plate at60° C. for 1 hour. The thus-treated substrate provided with theantireflection structure was used as a nanoimprinint stamper in thefollowing nanoimprininting process at room temperature. First, SOGmaterial (OCD-T12™, manufactured by Tokyo Ohka Kogyo Co., Ltd.) wasspin-coated on a 4-inch quartz substrate (synthesized quartz glass AQ™,manufactured by Asahi Glass Co., Ltd.) at 2000 rpm for 40 seconds, andbaked on a hot-plate at 110° C. for 90 seconds to evaporate the solventand thereby to form a SOG membrane. The stamper was nanoimprinted ontothe SOG membrane under 2.5 MPa for 1 minute at the substrate temperatureof 25° C., and then released. The pattern thus formed on the surface byimprinting was reverse to that of the antireflection structure formed byetching. The obtained substrate was annealed in air at 800° C. for 30minutes to convert SOG into SiO_(x), and thereby the mechanical strengthwas enhanced. Since having a small thermal shrinkage ratio, the patternobtained by imprinting the master plate onto the SOG membrane on thequartz substrate did not suffer pattern collapse even after the annealtreatment. The quartz substrate thus provided with the reverse patternwas then treated with a releasing agent, and was used as anano-imprinint stamper in the following nanoimprininting process. First,a glass substrate of 100 mm×100 mm was prepared. Then, polymethylmethacrylate (PMMA, Mw: 20000) was dissolved in PGMEA in 30 wt. %, andthe obtained solution was spin-coated on the substrate at 2000 rpm for40 seconds to form a 2.5 μm-thick PMMA membrane. The membrane wasthermally imprinted with the above stamper under 0.5 MPa for 2 minutesat the substrate temperature of 150° C. After the substrate was cooledto room temperature, the stamper was released. The structure thus formedon the PMMA surface was observed by SEM, and it was confirmed that theformed antireflection structure had almost the same pattern as themaster plate had.

The transmission and reflection spectra of the surface on which theantireflection structure was thus formed were measured. As a result, thetransmittances at the wavelengths of 470 nm, 550 nm and 630 nm werefound to be 99.3%, 99.5% and 99.5%, respectively. Accordingly, it wasfound that high transmittances were obtained in the whole visibleregion. On the other hand, the reflectances at the wavelengths of 470nm, 550 nm and 630 nm were found to be 0.6%, 0.40% and 0.4%,respectively. Accordingly, it was found that the reflectances werelargely reduced as compared with those (4.1%, 3.9% and 4.0%,respectively) before the antireflection structure was formed.

Comparative Example 1

The procedure of Example 1 was repeated to obtain a substrate providedwith a mono-particle layer formed on a SOG base layer. The SOG baselayer was processed by etching under the conditions that: the etchinggas was CF₄, the flowing amount was 30 sccm, the pressure was 0.01 Torr,RF power was 100 W and the etching time was 12 minutes. The etchingprocess was continued until all the silica particles serving as anetching mask completely disappeared. The obtained substrate was observedin detail by SEM. As a result, it was confirmed that the silicaparticles were slimed by etching and thereby that a pattern ofprojections was formed in the same manner as in Example 1. However, itwas also observed that 300% of the projections had tips sharpened byoveretching. The average interval among the projections, the averageheight of the projections and the aspect ratio of the projections were210 nm, 340 nm and 1.7, respectively.

The transmission and reflection spectra of the surface on which theantireflection structure was thus formed were measured. As a result, thetransmittances at the wavelengths of 470 nm, 550 nm and 630 nm werefound to be 99.1%, 98.90% and 99.00%, respectively. Accordingly, it wasfound that high transmittances were obtained in the whole visibleregion. On the other hand, the reflectances at the wavelengths of 470nm, 550 nm and 630 nm were found to be 0.9%, 1.0% and 1.0%,respectively.

The antireflection structure of the present invention can be applied forforming displaying surfaces of various instruments such as cellularphones, for producing surfaces of LCD guide plates and for manufacturingilluminating surface of lighting devices. Accordingly, the presentinvention can be favorably used for producing, for example, cellularphones, displays, lighting devices, watches, PCs, and music players.

1. A method for forming an antireflection structure in which pluralprojections are arranged on a substrate, comprising the steps of:forming a base layer on a substrate, forming a particle-trap layer onsaid base layer, forming, on said particle-trap layer, a multi-particlelayer in which particles having a mean particle size of 20 to 1000 nmare arranged in one or more layers, sinking the particles positioned atthe bottom of said multi-particle layer into the particle-trap layer,removing the particles in said multi-particle layer except the particlespositioned at the bottom, removing said particle-trap layer with holdingthe particles positioned at the bottom on the base layer, and performingan etching process in which said base layer is subjected to reactive ionetching while the particles remaining on said base layer are used as amask to form projections, under the conditions that the etching rateratio of said base layer to said particles is more than 1 but not morethan 5 and that the process is stopped before the particles disappear byetching where 80% or more of the particles remaining after the etchingprocess have a relative volume of more than 0% but not more than 25%based on the volume of those before the etching process, wherein saidparticles and said base layer are simultaneously etched to form anantireflection structure having a plurality of projections.
 2. Themethod according to claim 1, wherein said particles are made of silicamaterials and said base layer is made of at least one material selectedfrom the group consisting of silica materials, silicone materials andsilsesquioxane materials.
 3. The method according to claim 1, whereinsaid base layer and said particles are made of silsesquioxane materialsand silica materials, respectively, and the antireflection structureafter formed is further subjected to heat treatment at a temperature of400° C. to 1000° C.
 4. The method according to claim 1, wherein saidparticles are made of polystyrene resin and said base layer is made ofat least one material selected from the group consisting of acrylicresin, phenol resin, polystyrene resin and polyimide resin.
 5. Themethod according to claim 1, wherein said particles have a CV value of15% or less.
 6. The method according to claim 1, wherein the pluralityof projections are substantially conical projections.
 7. The methodaccording to claim 1, wherein after said etching process is performed,said plurality of projections comprise particles having a mean particlesize on the tips of said projections and the projections have roundtips.
 8. A method for forming an antireflection structure in whichplural projections are arranged, comprising the steps of: forming a baselayer on a substrate, forming a particle-trap layer on said base layer,forming, on said particle-trap layer, a multi-particle layer in whichparticles having a mean particle size of 20 to 1000 nm are arranged inone or more layers, sinking the particles positioned at the bottom ofsaid multi-particle layer into the particle-trap layer, removing theparticles in said multi-particle layer except the particles positionedat the bottom, removing said particle-trap layer with holding theparticles positioned at the bottom on the base layer, performing anetching process in which said base layer is subjected to reactive ionetching while the particles remaining on said base layer are used as amask to form projections, under the conditions that the etching rateratio of said base layer to said particles is more than 1 but not morethan 5 and that the process is stopped before the particles disappear byetching, so as to produce a master plate in which the plural projectionsare arranged on the substrate wherein 80% or more of the particlesremaining after the etching process have a relative volume of more than0% but not more than 25% based on the volume of those before the etchingprocess, wherein said particles and said base layer are simultaneouslyetched to form an antireflection structure having a plurality ofprojections, using said master plate to produce a nanoimprint stamperhaving a pattern reverse to the pattern formed on the master plate, andperforming a nanoimprinting process in which said nanoimprint stamper isused to produce a replica of the pattern on the master plate.
 9. Themethod according to claim 8, wherein said nanoimprint stamper or saidreplica is made of silsesquioxane materials, and said nanoimprintstamper or said replica after formed is further subjected to heattreatment at a temperature of 400° C. to 1000° C.
 10. The methodaccording to claim 8, wherein said particles have a CV value of 15% orless.
 11. The method according to claim 8, wherein the plurality ofprojections are substantially conical projections.
 12. A method forforming an antireflection structure in which plural projections arearranged, comprising the steps of: forming a base layer on a substrate,forming a particle-trap layer on said base layer, forming, on saidparticle-trap layer, a multi-particle layer in which particles having amean particle size of 20 to 1000 nm are arranged in one or more layers,sinking the particles positioned at the bottom of said multi-particlelayer into the particle-trap layer, removing the particles in saidmulti-particle layer except the particles positioned at the bottom,removing said particle-trap layer with holding the particles positionedat the bottom on the base layer, performing an etching process in whichsaid base layer is subjected to reactive ion etching while the particlesremaining on said base layer are used as a mask to form projections,under the conditions that the etching rate ratio of said base layer tosaid particles is more than 1 but not more than 5 and that the processis stopped before the particles disappear by etching, so as to produce amaster plate in which the plural projections are arranged on thesubstrate wherein 80% or more of the particles remaining after theetching process have a relative volume of more than 0% but not more than25% based on the volume of those before the etching process, whereinsaid particles and said base layer are simultaneously etched to form anantireflection structure having a plurality of projections, using saidmaster plate to produce a replica having a pattern reverse to thepattern formed on the master plate, using said replica to produce ananoimprint stamper having a pattern reverse to the pattern formed onthe replica, and performing a nanoimprinting process in which saidnanoimprint stamper is used to produce a replica of the pattern on themaster plate, using said master plate to produce a replica having apattern reverse to the pattern formed on the master plate, and usingsaid replica to produce a nanoimprint stamper having a pattern reverseto the pattern formed on the replica.
 13. The method according to claim12, wherein the plurality of projections are substantially conicalprojections.